1. Field of the Invention
The present invention relates generally to the measurement of viscosity of liquids, and in particular to electronic devices and methods which measure kinematic viscosity.
2. Background
Determining intrinsic viscosity and kinematic viscosity is a fundamental requirement to properly carry out many industrial and manufacturing processes, including food preparation. Generally, intrinsic viscosity is defined as the internal resistance to flow exhibited by a liquid. A liquid has an intrinsic viscosity of one poise if a force of one dyne per square centimeter causes two parallel liquid surfaces one square centimeter in area and one centimeter apart to move past one another at a velocity of one centimeter per second. Kinematic viscosity is equal to intrinsic viscosity divided by the density of the liquid, where both the intrinsic viscosity and density of the liquid are measured at the same temperature.
It is known that certain properties of liquids are related to the velocity of waveforms traveling through those liquids. One type of waveform is a longitudinal, or compressional wave. When an acoustic longitudinal wave travels through a liquid, the liquid molecules vibrate in the same direction as that in which the longitudinal wave is propagated. At certain frequencies of longitudinal waves, another type of waveform called a shear wave is also propagated in liquids. When a shear wave travels through a liquid, the molecules of the liquid vibrate in a direction transverse to the direction of propagation of the longitudinal wave. The vibration of the molecules induced by the shear wave is coupled to the natural frequencies of vibration of the liquid. As explained by A. B. Bhatia, Ultrasonic Absorption, Dover Publications, 1967, if the velocity of a shear wave is measured, the kinematic viscosity can be directly calculated, and furthermore, if the density of the liquid is also known, the intrinsic viscosity can be easily calculated.
Several known devices employ acoustic waves to measure the density of a liquid and use the density measurement to calculate or approximate the kinematic viscosity and intrinsic viscosity. These devices use acoustic waves to measure density, but do not directly measure the intrinsic viscosity or kinematic viscosity of a liquid.
One such device, described in U.S. Pat. No. 4,331,025, tries to measure the flow rate of a liquid by determining the velocity of a longitudinal wave propagated in the liquid based on the velocity of an acoustic transmission from an upstream location to a downstream location, and also from the downstream location to the upstream location. It then uses the temperature of the liquid and certain predetermined constants to calculate the kinematic viscosity of the liquid. This device and method measures the velocity of the longitudinal wave rather than the velocity of the corresponding shear wave. A disadvantage of this approach to the problem is that calculated empirical constants used to determine the kinematic viscosity of the liquid are dependent on the type of liquid to be measured. Thus, the ability to use this method on a wide variety of liquids is limited by the need to calculate the constants for each liquid prior to the use of the method and device on a new liquid.
Another device, described in U.S. Pat. No. 5,365,778, attempts to determine the intrinsic viscosity of the liquid in response to both longitudinal waves and shear waves. A disadvantage of this device is that, to perform the necessary calculations to determine kinematic viscosity, the device must measure the velocity of both shear wave and longitudinal waves, thus requiring multiple measuring transducers and additional data calculating elements.
Yet another device, disclosed in U.S. Pat. No. 3,903,732, attempts to determine liquid intrinsic viscosity and density by measuring the effective input impedance on a single transducer as a function of the liquid""s reactive dampening force. A disadvantage of this device is that it yields only the intrinsic viscosity and the density of the liquid, not the kinematic viscosity. A further disadvantage is hat six constants need to be calculated prior to using the device on the liquid of interest.
A difficulty encountered with some devices that use acoustic wave measurements when calculating kinematic viscosity and intrinsic viscosity is that they operate at frequencies above 1 MHz. At these high frequencies, the shear wave is not properly established due to the short transient time between the maxima and minima in the, waveform. Furthermore, at these. frequencies shear waves, which are highly attenuated, degenerate rapidly over a very short distance, thus making them difficult to detect. This prevents effective measurement of the shear wave velocity and use of shear waves to determine intrinsic viscosity and kinematic viscosity. As a result, a great deal of energy is required to propagate the shear wave for even short distances in a liquid, thus creating serious difficulties in efficiently measuring the velocity of the shear wave in industrial applications. A further problem is that the change of the liquid in reaction to a shear wave is a transient phenomena characterized by the relaxation time of the molecules as they respond to a wave. Consequently, it is desired that the acoustic wave be such that the relaxation time for the molecules in the liquid is long enough to significantly cause and maintain the shear wave.
In view of the above, it should be appreciated that there is a need for a device and method for determining the intrinsic viscosity and kinematic viscosity of a liquid which can be used with a variety of liquids without precalculation of constants. There is also a need for such a device that is capable of creating and maintaining a shear wave in a liquid and is capable/of measuring shear waves over an extended distance. There is a need for such a device to be able to measure shifts of shear wave velocity independent of shear wave amplitude. There is a further need for the device to have low power requirements and/or operate at a low frequency. The present invention satisfies these and other needs and provides further related advantages.
The present invention resides in a device and method for determining the intrinsic viscosity and kinematic viscosity of a liquid which can be used on a variety of liquids without precalculation of constants. It is capable of creating and maintaining a shear wave in a liquid and is capable of measuring shear waves over an extended distance. It also can measure shifts of shear wave velocity independent of shear wave amplitude. It has low power requirements and/or operates at a low frequency.
The acoustic viscometer can include an acoustic wave generator, a transmitting piezoelectric transducer, a receiving piezoelectric transducer, and a phase-shift detector. The acoustic wave generator and transmitting piezoelectric transducer, which is positioned so as to be in contact with a liquid, are operably connected to each other. The transmitting piezoelectric transducer can propagate a longitudinal wave through the liquid. The longitudinal wave and a corresponding shear wave can be detected by a receiving piezoelectric transducer, which is advantageously spaced-apart from the transmitting piezoelectric transducer, and is operably connected to a phase-shift detector. The phase-shift detector is also operably connected to the transmitting piezoelectric transducer. The phase-shift detector measures the difference in phase between the longitudinal wave propagated by the transmitting piezoelectric transducer and the longitudinal and shear wave detected by the receiving piezoelectric transducer. The difference in phase can be used to determine the velocity of the shear wave, and the kinematic and intrinsic viscosities of the liquid.
Some embodiments of the invention include a transmitting piezoelectric transducer which can propagate a shear wave in a liquid over a distance of at least about one inch. The transmitting piezoelectric transducer can be driven by a voltage of at least about ninety volts, and the transmitting piezoelectric transducer and/or the receiving piezoelectric transducer can have a coating which may have a thickness of less than 0.1 millimeter.
This invention can be used to determine the difference in phase between the longitudinal wave propagated by the transmitting piezoelectric transducer and the longitudinal and shear wave detected by the receiving piezoelectric transducer. This can be accomplished regardless of each wave""s amplitude.
In some embodiments of the invention, the transmitting piezoelectric transducer can propagate a shear wave with a frequency ranging between about 20 kHz and about 100 kHz. This advantageously allows the shear wave to be properly established. Some embodiments include a temperature sensor for measuring the temperature of the liquid. The acoustic wave generator may be a pulse generator or a crystal oscillator, and the acoustic viscometer may be battery powered.
The invention can include a phase-to-voltage convector connected to the phase-shift detector to convert the difference in phase between the longitudinal wave propagated by the transmitting piezoelectric transducer and the longitudinal and shear wave received by the receiving piezoelectric transducer to a voltage. An output device can connect to the phase-to-voltage convector and display a value corresponding to the voltage.
The method of the present invention may include placing the transmitting and receiving piezoelectric transducers in contact with the liquid. A longitudinal wave and a corresponding shear wave are propagated by vibrating the transmitting piezoelectric transducer. The receiving piezoelectric transducer detects the longitudinal and shear wave. The method also involves determining the difference in phase between the longitudinal wave propagated by the transmitting piezoelectric transducer and the longitudinal and shear wave detected by the receiving piezoelectric transducer, determining the velocity of the shear wave, and determining the kinematic and intrinsic viscosities of the liquid.
The methods of the present invention may also include allowing at least about one inch of spacing between the transmitting and receiving piezoelectric transducers, determining the difference in phase between the longitudinal wave propagated by the transmitting piezoelectric transducer and the longitudinal and shear wave detected by the receiving piezoelectric transducer regardless of wave amplitude, the transmitting piezoelectric transducer propagating a shear wave with a frequency between about 20 kHz to about 100 kHz, and determining the temperature of the liquid. The method may include liquid flowing past the transmitting and receiving piezoelectric transducers, and liquid located within a piece of fruit. Still other methods include converting the difference in phase between the longitudinal wave propagated by the transmitting piezoelectric transducer and the longitudinal and shear wave detected by the receiving piezoelectric transducer to a voltage, and transmitting the voltage to an output device for displaying a value that corresponds to the voltage.
One advantage of the invention is that it can provide a measurable shear wave for determining the intrinsic viscosity and the kinematic viscosity of a liquid. Thus, the acoustic viscometer provides an accurate measurement of the kinematic viscosity without any prior measurements of constants related to the subject liquid. An advantage of the invention is that it detects the phase shift of a shear wave without regard to attenuation of the shear wave, increasing the accuracy of the acoustic viscometer.
Other features and advantages of the present invention will be set forth in the description which follows and the accompanying drawings, wherein the preferred embodiments of the present invention are described and shown, and will become apparent to those skilled in the art upon examination of the following detailed description taken in conjunction with the accompanying drawings, may be learned by practice of the present invention.